1. Field of the Invention
The present invention relates to image forming method and apparatus for forming an image on a recording medium.
2. Related Background Art
A heat roller fixing device has widely been used with an image forming apparatus such as an electrophotographic printer and the like because it has high thermal efficiency and high safety. In such a fixing device, a surface temperature of a heat roller, a pressure between the heat roller and a pressure roller, and a width of a nip constitute important factors for determining the fixing ability of a toner image on a recording medium, the curling amount of the recording medium, and the temperature increase in the image forming apparatus. Now, the fixing ability and the curling amount have the contradictory tendency that when one of them is improved the other will be worsened. More specifically, in order to improve the fixing ability, it is preferable to increase the surface temperature of the heat roller and/or to increase the nip pressure and the nip width; however, by doing so the curling amount and the temperature in the apparatus will be increased (worsened). Among them, the increase in the surface temperature of the heat roller has the greatest detrimental effect upon the curling amount.
Further, the image forming apparatus has been made smaller and inexpensive, and a thickness and an outer diameter of the heat roller have been decreased. Recently, a heat roller having a diameter of 30 mm or less has been used. However, when the thickness of the heat roller is decreased or when the diameter of the heat roller is decreased, in order to increase the fixing ability of the toner image, it is more difficult to increase the nip width or nip pressure; thus, it is preferable to increase the surface temperature of the heat roller.
Consequently, as mentioned above, the curling amount of the recording medium and the temperature in the apparatus will be increased.
To avoid this, it is preferable to adjust the surface temperature in accordance with the atmospheric temperature during the fixing operation in order to give the minimum necessary heat quality to the recording medium, as disclosed in the Japanese Patent Publication No. 60-41354 (filed on Aug. 24, 1978, published on Sep. 17, 1985). Particularly, it is known that the fixing ability is greatly influenced upon the temperature of the recording medium and the surface temperature of the pressure roller. The temperature of the recording medium is governed by the atmospheric temperature, and the surface temperature of the pressure roller depends upon the time period elapsed from the initiation of energization of the image forming apparatus. Thus, in most cases, a temperature adjustment mode wherein the control of the surface temperature of the heat roller is changed on the basis of the detection of the atmospheric temperature includes the detection of the atmospheric temperature at the initiation of energization of the image forming apparatus. If the atmospheric temperature is lower than a predetermined temperature, the surface temperature of the heat roller is set to a value slightly higher than the normal value, and then, when the pressure roller is sufficiently warmed, the normal temperature adjustment mode is restored.
However, when the surface temperature is changed on the basis of the atmospheric temperature in this way, it is necessary to provide a special temperature detecting element for detecting the atmospheric temperature.
For example, in order to detect the atmospheric temperature, a temperature detecting element (for example, thermistor) for detecting the atmospheric temperature is arranged in place within the image forming apparatus, and the surface temperature of the heat roller of the heat roller fixing device is controlled on the basis of the detected temperature. In many cases, the temperature detecting element is disposed on a control board for controlling the operation of the image forming apparatus to eliminate the complexity of the wiring within the apparatus. However, since the temperature detecting element for detecting the atmospheric temperature is additionally provided, the apparatus becomes more expensive.
Further, in many cases, since the control board is disposed remote from the heat roller fixing device, the temperature detecting element does not correctly monitor the atmospheric temperature (including the temperature of the pressure roller) in the proximity of the heat roller fixing device. Thus, despite the pressure roller having been warmed adequately, if the atmospheric temperature in the proximity of the control board is low, the temperature detecting element for detecting the atmospheric temperature will judge or determine that the atmospheric temperature is low, with the result that the surface temperature of the heat roller is controlled to have a value higher than the normal value. This is unsuitable, particularly in an electrophotographic printer wherein a power source switch is frequently switched between an ON condition and an OFF condition to perform the switching of a print modes and the replacement of a photo-cartridge.